I have a .375JDJ in a G2 frame. Never had a misfire until I bought some Hornady .444 Marlin for reloading. I shot about 30 rounds and had 8-9 misfires that did go off the second time I pulled the trigger. I have shot this barrel a lot and it always worked perfectly with Remington cases. Any ideas what the problem might be? I also fired 15 rounds using Remington cases the same day and had no problem. Thanks for your help.

Did you load the rounds in question yourself? Same primers in all? Perhaps different primer pocket depths and some primer movement on the first pull? When this happens to me it's usually because I skipped a row when loading powder 😮

I once had something similar happen when shooting some of my first 460 handloads. The first hit would lightly dent the primer, but the second one would ignite it. Turns out I wasn't pressing the primers into the socket with enough force, and they weren't seating properly.

Make sure you seat your primers deeply enough. The fact that they fired the second time suggests they weren't seated as deeply as they should have been.

karl... all the loads were mine with all the same primers.

Lone wolf... all the primers were dented the first time around. This may be the problem. Next set of hand loads I'll make sure they're all seated well.
Thanks guys.

Even dented, it could still be related to headspace/die adjustment. Keep watching that as well....and compare rim thickness between brass brands to see if it can be the minor difference.
Bellmtcs website has a lot of info on different scenarios.

Are you sizing the cases so that they headspace off the shoulder, or relying only on the case rims to determine headspace?

KYODE and S4SU are probably on the right track based on my own 375 JDJ experiences. I have two 375 JDJ barrels and two sets of dies, both acquired used. It wasn't until recently that I purchased a Hornady headspace comparator and found that the old Pacific Durachrome die set was setting the shoulder back about .015 inch from the fired case measurements. The only reason these rounds were working was from the rim doing the headspacing. I have since adjusted the die to set the shoulder back about .003 from fired dimensions. Just throwing it out there.